I once had a roommate who was into drinking nothing but protein shakes out of something similar to this
free Nature's Plus shaker cup. One day, he made some liquid concoction, drank most of it, and then somehow dropped it behind his bed and forgot about it. A few weeks later, all four of us who lived in the apartment spend a Sunday afternoon cleaning it from top to bottom in an effort to get rid of the rank smell that had recently invaded our living space. We were a bit angry when we found out it was just his cup and that we'd just cleaned the apartment for nothing. So we threw a party to get the place messy again and all was solved.
[via the forums]